In this work we investigate network models that can be potentially employed in the simulation of scheduling algorithms for distributed computing applicati- ons. We seek to develop a model of TCP communication which is both high-level and realistic. Previous research works show that accurate and global modeling of wide-area networks, such as the Internet, faces a number of challenging issues. However, some global models of fairness and bandwidth-s- haring exist, and can be link with the behavior of TCP. Using both previous results and simulation (with NS), we attempt to understand the macroscopic behavior of TCP communications. We then propose a global model of the network for the Grid platform. We perform partial validation of this model in simulation. The model leads to an algorithm for computing bandwidth-sharing. This algorithm can then be implemented as part of Grid application simulations. We provide such an implementation for the simulation toolkit.